Axon guidance and synaptogenesis represent distinct biological processes in the development of a neuron, yet they are mediated by an overlapping set of extrinsic cues, including netrin, wnt, and semaphorin. In axon guidance, these cues modulate the formation of actin filaments (F- actin) into the filopodial extensions of the growth cone, whereas in synaptogenesis, they direct F-actin assembly at nascent presynaptic regions to scaffold the recruitment of the presynaptic assembly program. To ascertain how the same extrinsic cues instruct dissimilar actin dynamics when specifying axon guidance versus synaptogenesis, we will examine the mechanisms of actin regulation during these two processes in Caenorhabditis elegans. As C. elegans is translucent, its stereotyped axon morphology and synapse patterning are easily observed using cell- and synapse-specific fluorescent markers. Furthermore, the ease of conducting forward genetic screens, combined with the wealth of available characterized mutants, makes C. elegans a powerful genetic model for the study of neuronal development. We will first investigate the relative contribution of known actin regulators to axon guidance and synaptogenesis in a specific neuron. We hypothesize that subsets of actin regulators have an exclusive role in either axon guidance or synaptogenesis, and that the differential regulation and function of these actin regulators underlies the distinct actin dynamics involved in these two processes. In parallel, we will use a forward genetic approach to identify novel genes which link synapse-patterning cues to presynaptic F-actin assembly. Finally, by examining genetic interactions between actin regulators and the multiple extrinsic cues which mediate axon guidance and synaptogenesis in a single cell, we will uncover how these cues converge to modulate actin dynamics. Aberrant actin regulation in neurons has been linked to a number of neurological disorders, including Parkinson's disease and autism. A more complete understanding of the mechanisms of actin regulation in neurons is an important step in designing effective therapies for these conditions.